(a) Field of the Invention
The present invention relates to an illumination device, and more specifically to an illumination device so adapted as to permit obtaining a surface light source having uniform luminance.
(b) Description of the prior art
The conventional illumination devices comprise linear light source(s) 2 consisting of a single or plural fluorescent tubes, etc. arranged over a flat or curved reflecting plate 1 as shown in FIG. 1, and a diffusing plate 3 arranged on the side opposite to the linear light source 2.
Out of these illumination devices, the illumination device using a single linear light source provides such luminance distribution on the diffusing plate as shown in FIG. 2 on which luminance is the highest at the central area close to the light source 1 and luminance is gradually lowered toward the marginal portion, thereby making luminance non-uniform over the entire range of the diffusing plate 3. Especially when the light source 2 is brought closer to the diffusing plate 3 to design a this illumination device, luminance is especially enhanced at the area right over the light source 2, out of the areas of the diffusing plate 3, by the rays emitted upward from the light source 2 and attaining directly to the diffusing plate 3. For this reason, attempts have previously been made to obtain uniform luminance distribution on the diffusing plate by reducing the rays emitted upward from the light source.
As a conventional example accomplished by such an attempt, there is known the illumination device disclosed by Japanese Published Unexamined Utility Model Application No. 90106/61. In this example, as shown in FIG. 3 a plural number of linear light-shielding portions 4 having a definite width are arranged discontinuously on the top (close to the diffusing plate) of a fluorescent tube used as the linear light source. In this conventional example, the rays emitted upward from the light source are partially shielded by the light shielding portions and the rays to attain to the central area of the diffusing plate located right over the linear light source are reduced, thereby lowering luminance at the central area. However, luminance is still high at the outer areas neighboring the central area and luminance distribution is as shown in FIG. 4, whereby luminance distribution is not so uniform on the diffusing plate.
As another example of the illumination devices of this type, there is also known the illumination device disclosed by Japanese Published Unexamined Patent Application No. 133008/55. In this illumination device, arranged between the diffusing plate and the light source is a light quantity adjusting plate 6 on which light-shielding portions 5 such as black spots are distributed at an adequate density as shown in FIG. 5 for the purpose of obtaining relatively uniform luminance distribution by reducing the rays to reach the area at which luminance would be otherwise high.
This conventional illumination device can provide rather uniform luminance distribution on the diffusing plate, but requires delicate adjustment of the space between the light source and the light quantity adjusting plate at the assembly stage of the illumination device and therefore poses a relatively difficult problem to maintain high precision for the space. Further, when the illumination device comprises a plural number of light sources, distribution, density, etc. of the light-shielding portions must be varied on the light quantity adjusting plate in accordance with variation of luminance on the tube surfaces of the light source, thereby posing a difficult adjusting problem.